Speaking to an audience of around 100,000 at the Vatican, Francis said there were "contradictory reports" about the fate of the bishops and asked that "they be returned quickly to their communities".

On Tuesday, the "Oeuvre d'Orient" Christian association announced that the bishops had been released, but on Wednesday it backed away from the claim.

"Yesterday evening we received information from the Greek Orthodox Patriarchate questioning the release of the two bishops," said Catherine Baumont, spokeswoman for the group, which works to help Middle Eastern Christians.

"Unfortunately no tangible proof of the release has been obtained. The situation remains unclear, and we still don't know who took them," Baumont told Agence France Presse.

And a source in Aleppo's Greek Orthodox archdiocese told AFP it had no news on the fate of the bishops.

"We have no new information," said Ghassan Ward, a priest at the archdiocese.

"We can say that (as far as we know) they haven't been freed," he added.

Ward said there had been "no contact with them," adding that "efforts are continuing" to secure their release.

"We are very worried," he said.

The two men were traveling from the Turkish border when armed men intercepted the car they were in, forcing them out of the vehicle, Syrian state media and church sources reported.

The kidnappers were believed to be Chechen fighters, who stopped the car in an area outside of Aleppo, the church sources said.

"The news which we have received is that an armed group... (of) Chechens stopped the car and kidnapped the two bishops while the driver was killed," an official from the Syriac Orthodox diocese said in a statement posted online.

A source in the Greek Orthodox church said the kidnappers had described themselves as "Chechen jihadists."

The Syrian Observatory for Human Rights watchdog said Wednesday that the bishops had been kidnapped "in the region west of Aleppo, where a brigade of fighters from Dagestan is active."

"Four members of this brigade were killed in confrontations with rebels from the Free Syrian Army," Observatory director Rami Abdel Rahman told AFP.

The Syrian opposition has condemned the kidnapping, saying the rebel Free Syrian Army was not involved and pointing the finger at the Syrian regime.

"Efforts... to uncover the identities of the clerics' kidnappers and to liberate them indicate that the Syrian regime is responsible for the kidnapping, and (the) killing of Bishop Yohanna Ibrahim's driver," the opposition National Coalition said.

Christians account for around five percent of Syria's population, and have become increasingly vulnerable to attack and abductions in the lawlessness that has engulfed much of the country since March 2011.